This invention relates to a device for the automatic regulation of the position of a belt-tightening roller, during the operation of an internal combustion engine, said device being adapted to prevent the tension of the belt from undergoing any alteration relative to the value which was selected with the initial calibration due to the variations of the distance between the axes of the toothed pulleys which are integrally keyed to the timing shafts and the driving shaft, respectively, as caused by expansions and contractions of the engine block.
The toothed belts as employed in transferring the drive from the driving shaft to the timing mechanism shafts are generally so constructed (with a core of glass fiber or of a spirally wound steel wire) that, after an initial settling elongation which is the result of a plastic deformation, are virtually indeformable and unextensible, whereas the engine block and the engine head, especially, when they are made of a lightweight alloy, that is a material which is characterized by a high expansion coefficient, such as for example aluminum, undergo considerable expansions and contractions due to the effect of temperature differentials to which they are subjected during the engine operation and during the cooling thereof which is a result of the engine stoppage, especially when the ambient temperature is extremely low. Due to the variations of the distances between the axles, modifications are experienced, which are even most significant, in the initial belt tension, so that troubles in operation can be experienced in the operation of the belt and the engine performance could even be negatively influenced.
As the tension is increased, the operation of the belt becomes much more noisy and, while at the high rpm's of the engine the noise caused by a belt which operates incorrectly can be exchanged for other noises as originated by the engine and the several moving members, at the low rpm's such a noise is discretely perceptible relative to other noises, the latter being dampened to a degree under such conditions.
The increased noise due to the increase of the belt tension is presumably due to the fact that the meshing between the belt teeth and the pulley teeth takes place with more abrupt bumps, but also to the fact that during the approaching stage the air is ejected rapidly and completely from the inter-tooth cavities, since the sides of the 'belt teeth forcibly stick to the sides of the pulley teeth, whereas, during the detachment stage, an abrupt air draw is experienced towards the intertooth hollows. The latter speculation seems to have been corroborated by the fact that the incorrect operation of the belt is accompanied by a swishing sound which can neatly be distinguished among the other noises of the mechanical component parts.
A decrease of the belt tension is as much detrimental even though due to different reasons. The belt stays slacking on the pulleys and backlashes can be originated between the belt teeth and the pulley teeth, which can give rise to a tooth skip, especially when the engine is started. The phase change of the timing shafts is a result, relative to the driving shaft, with a serious jeopardy for the correct engine operation.
With the chain transfer mechanisms, a problem arises, which, under a few aspects, is akin to that which has been outlined hereinabove in connection with the toothed belt transfer mechanisms. During the engine service life, the chain becomes elongated due to the effect of stresses which originate permanent deformations, so that the chain tension shows a tendency towards being diminished relative to its initial magnitude.
In order to maintain a chain at the designed pull, devices are generally employed, which are generally composed by a pad kept into contact with the driving lap of the chain by specially provided mechanical or hydraulic actuators. Every time that a backlash takes place between the pad and the chain due to the effect of an elongation of the latter, the pad is brought back to contact the chain so as to restore the initial tension again.
These devices, however, are of the unidirectional action type, in the sense that they adjust the chain tension only in the case that the chain is slackened. The pad, then, remains latched in its outermost position until a subsequent action is taken.